Escherichia coli genes that reduce the lethal effects of stress

Abstract

Background: The continuing emergence of antimicrobial resistance requires the development of new compounds and/or enhancers of existing compounds. Genes that protect against the lethal effects of antibiotic stress are potential targets of enhancers. To distinguish such genes from those involved in drug uptake and efflux, a new susceptibility screen is required.

Results: Transposon (Tn5)-mediated mutagenesis was used to create a library of Escherichia coli mutants that was screened for hypersensitivity to the lethal action of quinolones and counter-screened to have wild-type bacteriostatic susceptibility. Mutants with this novel "hyperlethal" phenotype were found. The phenotype was transferable to other E. coli strains by P1-mediated transduction, and for a subset of the mutants the phenotype was complemented by the corresponding wild-type gene cloned into a plasmid. Thus, the inactivation of these genes was responsible for hyperlethality. Nucleotide sequence analysis identified 14 genes, mostly of unknown function, as potential factors protecting from lethal effects of stress. The 14 mutants were killed more readily than wild-type cells by mitomycin C and hydrogen peroxide; nine were also more readily killed by UV irradiation, and several exhibited increased susceptibility to killing by sodium dodecyl sulfate. No mutant was more readily killed by high temperature.

Conclusions: A new screening strategy identified a diverse set of E. coli genes involved in the response to lethal antimicrobial and environmental stress, with some genes being involved in the response to multiple stressors. The gene set, which differed from sets previously identified with bacteriostatic assays, provides an entry point for obtaining small-molecule enhancers that will affect multiple antimicrobial agents.

Figure 1

Antimicrobial susceptibilities of insertion mutants. E. coli cultures grown to mid-log phase were treated with various concentrations of antimicrobial agents for 2 hr at 37°C. Bactericidal activity was expressed as percent survival relative to the CFU per ml at the time of drug addition. The concentration that reduced CFU by 90% was taken as LD₉₀. The values are the means of 3 independent experiments. Error bars indicate standard deviations of means.

Figure 2

Susceptibilities of insertion mutants to physical and chemical stresses. E. coli cultures grown to mid-log phase were treated with 2000 μJ/cm² of UV; 2 mM H₂O₂, 10% SDS, or heat shock at 52°C for 15 min. Samples were diluted, applied to agar lacking stressor, and incubated to determine the fraction of colonies surviving. This fraction was expressed as a percent of an untreated control culture sampled at the time stress was applied. In the case of SDS, some mutants grew during treatment, which caused those samples to have values higher than the control. Values reported are the means of 3 independent experiments. Error bars indicate standard deviations of means.

Figure 3

Complementation of hyperlethal phenotype by cloned genes. Plasmids containing wild-type genes were transformed into the corresponding Tn5-containing mutants. The strains harboring the plasmids were then tested for nalidixic acid-mediated lethality by treating mid-log phase cells with various concentrations of nalidixic acid for 2 hr at 37°C. Percent of control indicates percent survival of treated cells relative to untreated cells sampled at the time of drug addition. For ycjW, yrbB, and ybcM, the expression was induced by adding 1 mM of IPTG 2 hr before nalidixic acid treatment. Similar results were obtained in a replicate experiment.